Hi there! Chemistry background and Salon Owner here. This is a multifaceted question as alot of things that can be potentially be bad can also be potentially beneficial. When discussing cosmetic formulation you could equate it to that of baking a cheesecake. If you use the right amount of sugar a cheesecake will have perfect consistency, if you use too much it will become caramelized resulting in a different end result. This can also be used in theory and practically when it comes to products as lone use versus pairing them with other ingredients. A few things that pose risks to the scalp and hair would be the following. Sulfates, parabens, silicones, oils and fragrances. Sulfates can be very irritating to the scalp, but are very necessary for consumers who do not wash regularly due to their job is to break down oil and grime. Consistent use of harsh chemicals can cause contact dermatitis especially in people who has sensitive skin or skin issues like psoriasis. That said sulfate molecules can be designed to be less harsh, but you need to do your due diligence when using them. Parabens not only can cause contact dermatitis but they are also an endocrine disruptor which is something to note being that females are the leading gender demographic in beauty product consumption. Fragrances are probably the most dangerous in the list being that companies can get around alot by putting in chemicals without disclosing what is inside the scent under the guise of trade secrets. They can cause allergic reactions, scalp inflammation, hormonal disruption due to common use of Phthalates amongst many other potential side effects due to fragrance. Oil would be another one i tell all my clients to avoid as many of the oiling scalp and oil based product trends do not have much benefit unless they are fractured/high grade. Most clients use oil based products that contain food grade coconut oil which actually has a fatty molecule that is too big to penetrate the hair and will sit on your scalp and clog poors. Also people tend to forget that oil and water are immiscible and oil will push water way from your hair. Oil should be used as a finishing product not as a cleansing aid or on wet hair. Silicones are both good and bad. They are rates on a measurement called a centistoke. the lower the "cst" the more it acts like water and is high grade, the higher it is the more it acts like sticky plastic and will cause buildup. So the same silicone can be bad or good based on quality.
Certain hair care products contain harmful ingredients that can negatively impact scalp and hair health. Sulfates, commonly found in shampoos, are effective in cleansing but can strip natural oils, leading to dryness. Insights from trichologists and dermatologists highlight the risks associated with these ingredients, emphasizing the importance of choosing products that prioritize scalp and hair wellness.